Modern vehicles use three-way catalysts (TWC) for exhaust after-treatment of gasoline engines. With tightening government regulations on automobile emissions, feedback control is used to adequately regulate the engine air-to-fuel ratio (AFR). Some vehicles have a universal exhaust gas oxygen (UEGO) sensor upstream of the TWC and a heated exhaust gas oxygen (HEGO) sensor downstream of the TWC to control the AFR near stoichiometry. This is achieved by regulating the AFR to a set point around stoichiometry, which in turn is fine-tuned based on the deviation of a HEGO voltage from a pre-determined HEGO-voltage set-point.
However, various faults, such as AFR imbalance between cylinders, could bias the UEGO sensor reading rich or lean of stoichiometry. This can lead to significant feedgas emissions such as carbon monoxide (CO) or the oxides of nitrogen (NOx) passing directly to the tailpipe, as the biased air/fuel mixture is fed directly to the catalyst, overwhelming the oxygen-storage buffer that allows for short deviations from stoichiometry. These asymmetric faults may be caused, for example, by a degraded UEGO sensor, cylinder imbalance resulting from a degraded fuel injector, or an error incurred during a deceleration fuel shutoff event. Detecting and correcting for asymmetric biasing may include first running an intrusive diagnostics test, thereby increasing the risk of generating significant tailpipe emissions in the presence of an existing biasing fault.
The inventors herein have recognized the above issue and have devised various approaches to address it. In particular, systems and methods for identifying and rejecting asymmetric faults that cause engine emissions to be biased rich or lean are disclosed. In one example, a method for an engine system comprises: generating a UEGO sensor feedback set-point adjustment based on slower and faster time components within an outer loop of a catalyst control system; generating an inner-loop bias-offset correction from the slower time component; and indicating degradation of the engine system based on a comparison of the bias-offset correction to a degradation threshold. In this way, the total outer-loop control authority is increased while maintaining drivability and noise, vibration, and harshness (NVH) constraints and meeting emission standards in the presence of an air-to-fuel ratio biasing fault.
In another example, a method for controlling an internal combustion engine having an upstream exhaust gas sensor positioned upstream relative to a catalyst and a downstream exhaust gas sensor positioned downstream relative to a catalyst, comprises: generating an upstream exhaust gas sensor feedback set-point adjustment based on a downstream exhaust gas sensor feedback signal; monitoring the upstream exhaust gas sensor bias offset for a constant or slowly-varying bias; generating a bias-offset correction responsive to the constant or slowly-varying bias; adjusting the downstream exhaust gas sensor feedback signal with the bias-offset correction responsive to a temporal event. In this way, the generation of tailpipe emissions in the presence of an asymmetric biasing fault may be prevented.
In another example, a system for controlling an internal combustion engine, comprises: a first exhaust gas oxygen sensor positioned downstream relative to the engine; a catalyst positioned downstream relative to the first exhaust gas sensor; a second exhaust gas oxygen sensor positioned downstream relative to the catalyst; a controller in communication with the first and second exhaust gas oxygen sensors, the controller comprising an inner feedback control loop to control air-fuel ratio of the engine with feedback provided via the first exhaust gas oxygen sensor and an outer feedback control loop that modifies a reference air-fuel ratio provided to the inner feedback control loop based on feedback from the second exhaust gas oxygen sensor wherein the controller monitors the reference air-fuel ratio over time for a constant or slowly-varying bias and corrects the reference air-fuel ratio responsive to the constant or slowly-varying bias; and where the controller disables monitoring the reference air-fuel ratio for a pre-determined amount of time responsive to a deceleration fuel shutoff event. In this way, asymmetric biasing faults may be properly identified and rejected without the need for intrusive diagnostics tests.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.